1. Field of the Invention
The present invention relates to a piezoelectric thin-film resonator and a filter using the same, and more particularly, to a piezoelectric thin-film resonator having a space located below a resonance portion in which an upper electrode and a lower electrode face each other across a piezoelectric film and a filer using the same.
2. Description of the Related Art
There has been an increasing demand for compact and lightweight resonators and filters using such resonators due to rapid spreading of wireless equipment such as cellular phones. In the past, dielectric filters and surface acoustic wave (SAW) filters were used. Recently, there has been a considerable activity in the research and development of a piezoelectric thin-film resonator that can be miniaturized and monolithically manufactured and a filter using such a resonator.
An FBAR (Film Bulk Acoustic Resonator) type resonator is known as one of the piezoelectric thin-film resonators. The FBAR has a film laminate composed of an upper electrode, a piezoelectric film and a lower electrode. A space, which may be a via hole or cavity, is provided below the lower electrode and located within an overlapping region (resonance portion) in which the upper and lower electrodes overlap with each other across the piezoelectric film. The space may be formed below a dielectric film provided under the lower electrode. The via hole may be defined by wet-etching a silicon substrate that may be used as a device substrate from the backside of the silicon substrate. The cavity may be defined by forming the resonator composed of the film laminate on a sacrificed layer on the surface of the substrate and removing the sacrificed layer. In this manner, the piezoelectric thin-film resonators are of via-hole type and cavity type.
A high-frequency signal is applied between the upper electrode and the lower electrode, an acoustic wave is generated within the piezoelectric film sandwiched between the upper and lower electrodes. The acoustic wave thus generated is excited by the reverse piezoelectric effect and distortion arising from the piezoelectric effect. The acoustic wave is totally reflected by the surface of the upper electrode (film) that is in contact with air and the surface of the lower electrode (film) that is in contact with air. Thus, the acoustic wave is a thickness-extensional wave having main displacements in the thickness direction. In the present device structure, a resonance takes place at a frequency at which the total thickness H of the thin-film laminate structure composed of the upper electrode/piezoelectric film/lower electrode located above the opening is equal to an integer multiple of the ½ wavelength of the acoustic wave. The propagation velocity V of the acoustic wave is determined by the material used, and the resonant frequency F is written as F=nV/2H. By utilizing the resonant phenomenon, it is possible to control the resonant frequency with the thickness being used as a parameter for control and to realize the resonators and filters having desired frequency responses.
The upper and lower electrodes may be a film laminate made of a metal such as aluminum (Al), copper (Cu), molybdenum (Mo), tungsten (W), tantalum (Ta), platinum (Pt), ruthenium (Ru), rhodium (Rh), iridium (Ir), chromium (Cr), or titanium (Ti), or an arbitrary combination of these metals. The piezoelectric film may be aluminum nitride (AlN), zinc oxide (ZnO), lead zirconium titanate (PZT), or lead titanate (PbTiO3). Preferably, the piezoelectric film is aluminum nitride or zinc oxide having an orientation axis in the (002) direction. The device substrate may be made of silicon (Si), glass or gallium arsenide (GaAs).
As described above, the piezoelectric thin-film resonator having the above-mentioned structure is required to have a via hole or cavity just below the lower electrode (or a dielectric film). In the following description, the via hole is defined as a hole that penetrates the device substrate and connect the upper and lower surfaces thereof, and the cavity is defined as a hole provided in the vicinity of the surface or just below the lower electrode.
FIG. 1 is a cross-sectional view of a conventional piezoelectric thin-film resonator described in Electron. Lett., 1981, vol. 17, pp. 507-509 (hereinafter referred to as Document 1). A laminate structure is provided on a (100) silicon substrate 11 having a thermal oxide film (SiO2), and is composed of a lower electrode 13 that is an Au—Cr film, a piezoelectric film 14 that is a ZnO film, and an upper electrode 15 that is an aluminum film. A via hole 16 is formed below the film laminate. The via hole 16 may be formed by anisotropic etching using KOH or EDP (ethylenediamine and pyrocatechol) from the backside of the (100) silicon substrate 11.
FIG. 2 is a cross-sectional view of a conventional piezoelectric thin-film resonator of the cavity type disclosed in Japanese Patent Application Publication No. 60-189307 (Document 2). A film laminate is composed of a lower electrode 23, a piezoelectric film 24 and an upper electrode 25 provided on a device substrate 21 having a thermal oxide film (SiO2) 22. A cavity 26 is defined below the film laminate. The cavity 26 may be formed by patterning an island-like sacrificed layer of ZnO, forming the film laminate on the sacrificed layer, and removing the sacrificed layer located below the film laminate by acid.
The piezoelectric thin-film resonators mentioned above have resonance portions in which the upper electrodes 15 and 25 face the lower electrodes 13 and 23 across the piezoelectric films 14 and 24. Vibration energy is confined in the resonance portions so that a high quality factor Q can be realized. Japanese Patent Application Publication No. 2006-128993 (Document 3) shows a technique shown in parts (a) through (c) of FIG. 3. Vibration media (which include a lower electrode 33, a piezoelectric film 34 and an upper electrode 35) in the vicinity of a resonance portion 37 formed on a substrate 31 having a cavity 36. Removal of the vibration media increases a high quality factor Q.
Even in the resonator disclosed in Document 2, vibration energy may be scattered and lost through an upper electrode 35b in an interconnection portion 38 for extracting a signal from an upper electrode 35a of the resonance portion 37 and through the piezoelectric film 34 below the upper electrode 35b. This degrades the quality factor Q. Removal of the piezoelectric film 34 in the interconnection portion 38 restrains scattering and losing of the vibration energy of the resonance portion 37. However, the removal of the piezoelectric film 34 in the interconnection portion 38 complicates the manufacturing process of the piezoelectric thin-film resonator.